Gel Coat
- Primarily aesthetic
- Color, texture, finish
- Does not add structural strength
A common surface finish for FRP composites is gel coat. A gel coat is a specially formulated polyester resin incorporating thixotropic agents to increase the gel coat’s viscosity and non-sag properties, fillers for flow properties, pigments to give the desired color, and additives for specific application properties, such as gel time and cure. Gel coats are primarily used for contact molding (hand or spray lay-up). The gel coat, usually pigmented, provides a molded-in finished surface that is weather and wear resistant. The gel coat helps in hiding the glass reinforcement pattern that may show through from the inherent resin shrinkage around the glass fibers. Gel coats are typically applied by spray application to approximately 16-20 mils (0.003 mm) wet film thickness. While gel coats do not add any structural strength to the composite part, gel coats should be resilient. Gel coats should be able to bend without cracking. They should be resistant to thermal cracking (cracking that may occur with dramatic changes in temperature). Gel coat is not paint. Paint contains solvents that must evaporate for the paint to dry. Gel coat does not have a solvent base; it has instead a reactive diluent called a monomer, which cross-links during curing.
Surface Veils
- Aesthetic value
- Weather barrier
- Abrasion resistance
- Inhibit corrosion
- UV protection
In some composite designs, a surface veil is used to provide an improved corrosion or weather barrier to the product. A surface veil is a fabric made from nylon or polyester that acts as a very thin sponge that can absorb resin to 90% of its volume. This helps to retain an extra layer of protective resin on the surface of the product. Surface veils are used to improve the surface appearance and ensure the presence of a corrosion resistance barrier for typical composites products such as pipes, tanks and other chemical process equipment. Other benefits include increased resistance to abrasion, UV and other weathering forces. Veils may be used in conjunction with gel coats to provide reinforcement to the resin.
Adhesives
bonding composites to composites or other materials
- Epoxy – high temperature resistance
- Acrylic – strength, rapid cure
- Urethane – high strength
Adhesives are used to attach composites to themselves as well as to other surfaces. Adhesive bonding is the method of choice for bonding thermoset composites and is sometimes used for thermoplastic composites. Adhesives should be used in a joint design where the maximum load is transferred into the component using the loading characteristics of the adhesive and the composites material. The most common adhesives are acrylics, epoxies, and urethanes. A high-strength bond with high-temperature resistance would indicate the use of an epoxy, whereas a moderate temperature resistance with good strength and rapid cure might use an acrylic. For applications where toughness is needed, urethane might be selected.
Painting
bonding composites to composites or other materials
- Aesthetic value
- UV protection
Painting systems are available for FRP composite finishes and are widely used in both the architectural and marine fields. Properly prepared composites can also accept a wide variety of surface coatings, including oil- and water-based paint, as well as plural component systems such as urethanes. The non-absorbent, inert nature of FRP composites allow for surface paints to be applied. Paints need not be breathable and no extraordinary surface preparation is required beyond proper abrasion and removal of residual mold release agents. Ceramic tile, metal, wood, and other plastics can be adhered to composite surfaces, provided differential thermal properties and panel deflections are allowed for either in the elasticity of the adhesive or mechanical attachment.
